Pollution control devices are used on motor vehicles to reduce atmospheric pollution. Two types of devices are currently in widespread use: catalytic converters and diesel particulate filters or traps. Catalytic converters contain one or more catalysts, which are typically coated onto a substrate in the form of a monolithic structure. The monolithic structures are typically ceramic, although metal monoliths have been used. The catalyst(s) oxidize carbon monoxide and hydrocarbons, reduce the oxides of nitrogen in exhaust gases, or a combination thereof. Diesel particulate filters or traps typically are in the form of wall flow filters that have honeycombed monolithic structures made from porous crystalline ceramic materials. In the current state-of-the-art construction of these pollution control devices, the monolithic structure of each type is enclosed within a housing.
A typical monolithic structure generally has relatively thin walls to provide a large amount of surface area. Consequently, the structure is fragile and susceptible to breakage. The typical monolithic structure formed from ceramic material tends to have a coefficient of thermal expansion that is an order of magnitude less than the metal (usually stainless steel) housing in which it might be contained. Protective packing materials such as ceramic mats or paste materials are typically packed between the ceramic monolith and the metal housing to avoid damage to the monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing. The process of placing or inserting the protective packing material is referred to as “canning” and includes such processes as injecting a paste into a gap between the monolith and the metal housing, or wrapping a sheet material (i.e., mounting mat) around the monolith and inserting the wrapped monolith into the housing.
Typically, the compositions used to form conventional protective packing materials include refractory ceramic fibers that provide properties such as high temperature durability, good handling, resiliency, flexibility, and strength. An intumescent material can also be included that enables the protective packing materials to volumetrically expand at elevated temperatures. Such expansion helps to hold the monolith in place during use. Ceramic sheet materials, ceramic pastes, and intumescent sheet materials useful for mounting the monolith in the housing of a pollution control device are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), and U.S. Pat. No. 5,242,871 (Hashimoto et al.).